The present invention relates to optical sensors, particularly but not exclusively to optical sensors based on planar waveguide gratings for sensing the refractive index of fluid samples and parameters and characteristics related thereto.
Measurement of the optical refractive index of fluids is of importance in fields including bioanalysis and biosensing. Different fluids, or concentrations of the same fluid, have different refractive indices so that the fluids can be identified or distinguished from refractive index measurements. The refractive index or change in refractive index of a fluid can be used to determine many biologically important measurands such as protein concentrations and glucose levels. In other fields, refractive index measurements are used in applications as diverse as process control and the detection of explosives [1].
Various devices and techniques for measuring refractive index are known. These include the Abbe-type refractometer, and sensors based on surface plasmon resonance. Optical waveguides may also be used. The presence of a liquid adjacent to an optical waveguide can alter the effective modal index of light propagating within the waveguide. This modification of index can be measured using techniques that are sensitive to changes in optical path length. For example, interferometer structures have been used to measure index changes and hence to sense the presence of proteins [2].
An alternative to interferometric measurement is to use optical waveguide gratings, where the reflective properties of a grating, which depend on refractive index, are modified by the index of a liquid in contact with the waveguide. An early device of this type used planar optical waveguides with surface relief gratings, in which adsorption and desorption of gas on the waveguide surface changed the refractive index and allowed the gas to be sensed [3]; other planar grating configurations have been proposed for integrated optical biological and chemical sensors [4]. Other grating-based devices have used fibre Bragg gratings [5]. Proposals have included thinning of the fibre with wet-etching to increase sensitivity [6], and the use of long period gratings in D-fibre, found to be more sensitive than gratings in standard telecom fibre [7]. However, the use of fibre gratings requires tedious working of the fibre to expose the waveguiding core to the liquid, such as by etching, which is difficult to control in a fibre geometry, or mounting the fibre in a block and polishing it down to access the core. Planar waveguide implementations are preferable in that it is typically easier to gain access to the propagating optical mode.